Separation of multi-component fluid through ultrasonic acoustophoresis

ABSTRACT

An acoustic standing wave is utilized to separate components from a multi-component fluid, such as oil from an oil-water mixture, in a fluid flow scheme with an acoustophoresis device. For example, the flow scheme and device allows for trapping of the oil as the oil coalesces, agglomerates, and becomes more buoyant than the water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/943,529, filed Jul. 16, 2013, which claims priority to U.S.Provisional Patent Application Ser. No. 61/671,856, filed on Jul. 16,2012; and is also a continuation-in-part of U.S. Ser. No. 13/844,754,filed Mar. 15, 2013, which claimed the benefit of U.S. ProvisionalPatent Application Ser. No. 61/611,159, filed Mar. 15, 2012, and of U.S.Provisional Patent Application Ser. No. 61/611,240, also filed Mar. 15,2012, and of U.S. Provisional Patent Application Ser. No. 61/754,792,filed Jan. 21, 2013. These applications are incorporated herein byreference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT

This invention was made with U.S. government support under Phase 1 SBIRAward No. IIP-1215021 awarded by the National Science Foundation. TheU.S. government has certain rights in the invention.

BACKGROUND

Many industrial applications generate wastewater that is contaminatedwith undesirable or hazardous fluid materials, such as oil. Theseoperations include oil drilling, mining and natural gas fracking. Also,spills from oil rigs into seawater generate emulsified oil in the waterthat is difficult to separate. The use of methods such as hydrocyclones,absorptive media, mechanical filtration, and chemical dispersion toseparate the oil from the water are both cost prohibitive and possiblyinjurious to the environment.

Acoustophoresis is the separation of particles using high intensitysound waves. It has long been known that high intensity standing wavesof sound can exert forces on particles. A standing wave has a pressureprofile which appears to “stand” still in time. The pressure profile ina standing wave varies from areas of high pressure (nodes) to areas oflow pressure (anti-nodes). Standing waves are produced in acousticresonators. Common examples of acoustic resonators include many musicalwind instruments such as organ pipes, flutes, clarinets, and horns.

It would be desirable to provide more effective methods of separatingemulsified oil and other contaminants from the contaminated water atreduced cost and low environmental impact.

BRIEF DESCRIPTION

The present disclosure relates to systems and devices foracoustophoresis on preferably a large scale. The devices use one or moreunique ultrasonic transducers as described herein, or an array of suchtransducers. The transducer is driven at frequencies that producemulti-dimensional standing waves.

Disclosed in certain embodiments is an acoustophoresis device,comprising: one or more device inlets at a first end of the device, thefirst end having a first diameter for receiving fluid flow; a contouredwall downstream of the inlet that narrows the fluid flow to a seconddiameter of a connecting duct; a flow chamber downstream of theconnecting duct, the flow chamber having: an inlet at a first end forreceiving the fluid flow, an outlet at a second end opposite the firstend, at least one ultrasonic transducer located on a wall of the flowchamber, the ultrasonic transducer including a piezoelectric materialdriven by a voltage signal to create a multi-dimensional standing wavein the flow chamber, and a reflector located on a wall on the oppositeside of the flow chamber from the at least one ultrasonic transducer; afirst device outlet located at the first end of the device and separatedfrom the device inlet by a longitudinal sidewall; and a second deviceoutlet located at a second end of the device downstream of the flowchamber outlet.

The device may include a plurality of device inlets spaced about thefirst end of the device, and the longitudinal sidewall is spaced apartfrom the contoured wall.

The piezoelectric material of the at least one ultrasonic transducer canhave a rectangular shape. The reflector can have a non-planar surface.

In particular embodiments, the first end of the device has a circularcross-section and the flow chamber has a rectangular cross-section.

The multi-dimensional standing wave generated by the transducer canresult in an acoustic radiation force having an axial force componentand a lateral force component that are of the same order of magnitude.

In embodiments, the transducer comprises: a housing having a top end, abottom end, and an interior volume; and a crystal at the bottom end ofthe housing having an exposed exterior surface and an interior surface,the crystal being able to vibrate when driven by a voltage signal.

Sometimes, no backing layer is present within the housing, and an airgap is present in the interior volume between the crystal and a topplate at the top end of the housing.

In other devices, the transducer further comprises a backing layercontacting the interior surface of the crystal, the backing layer beingmade of a substantially acoustically transparent material. Thesubstantially acoustically transparent material can be balsa wood, cork,or foam. The substantially acoustically transparent material may have athickness of up to 1 inch.

The flow chamber can further comprise a transparent window for viewingthe interior of the flow chamber.

In particular embodiments, the device has a length L from the at leastone device inlet to a bottom of the longitudinal sidewall, and a ratioof the length L to the first diameter is less than 1.

These and other non-limiting characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a front top perspective view of an exemplary embodiment of adevice of the present disclosure.

FIG. 2 is a front bottom perspective view of the device of FIG. 1.

FIG. 3 is a right side view of the device of FIG. 1.

FIG. 4 is a front view of the device of FIG. 1.

FIG. 5 is a rear view of the device of FIG. 1.

FIG. 6 is a left side view of the device of FIG. 1.

FIG. 7 is a top view of the device of FIG. 1.

FIG. 8 is a bottom view of the device of FIG. 1.

FIG. 9 is a right side cross-sectional view of the device of FIG. 1.

FIG. 10 is a cross-sectional diagram of a conventional ultrasonictransducer.

FIG. 11 is a cross-sectional diagram of an ultrasonic transducer of thepresent disclosure. An air gap is present within the transducer, and nobacking layer is present.

FIG. 12 is a photo of a square transducer and a circular transducersuitable for use in the devices of the present disclosure.

FIG. 13 is a graph of electrical impedance amplitude versus frequency asa square transducer is driven at different frequencies.

FIG. 14 illustrates the trapping line configurations for seven of thepeak amplitudes of FIG. 13.

FIG. 15 illustrates a possible array configuration for a group oftransducers.

FIG. 16 illustrates another possible array configuration for a group oftransducers.

FIG. 17 is a computer model of an acoustophoretic separator simulated togenerate FIGS. 18-29.

FIG. 18 shows a simulation of the axial forces on a particle in anacoustophoretic separator having a piezoelectric crystal producing asingle standing wave.

FIG. 19 shows a simulation of the lateral forces on a particle in anacoustophoretic separator having a piezoelectric crystal producing asingle standing wave.

FIG. 20 shows a simulation of the axial forces on a particle in anacoustophoretic separator having a piezoelectric crystal in a multi-modeexcitation.

FIG. 21 shows a simulation of the lateral forces on a particle in anacoustophoretic separator a piezoelectric crystal in a multi-modeexcitation.

FIG. 22 shows a three dimensional computer generated model of a modeshape calculation for a circular crystal driven at a frequency of 1 MHz.

FIG. 23 shows the lateral (horizontal) acoustic radiation force at1.9964 MHz.

FIG. 24 shows the axial (vertical) component for a resonance frequencyof 1.9964 MHz.

FIG. 25 shows the acoustic pressure amplitude at 1.9964 MHz.

FIG. 26 shows the lateral force component at a resonance frequency of2.0106 MHz.

FIG. 27 shows the axial acoustic radiation force component at aresonance frequency of 2.0106 MHz.

FIG. 28 shows the lateral force component at a resonance frequency of2.025 MHz.

FIG. 29 shows the axial acoustic radiation force component at aresonance frequency of 2.025 MHz.

FIG. 30 is a picture showing the results of an oil/water separationexperiment.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference tothe following detailed description of desired embodiments and theexamples included therein. In the following specification and the claimswhich follow, reference will be made to a number of terms which shall bedefined to have the following meanings.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in the specification and in the claims, the term “comprising”may include the embodiments “consisting of” and “consisting essentiallyof.”

Numerical values should be understood to include numerical values whichare the same when reduced to the same number of significant figures andnumerical values which differ from the stated value by less than theexperimental error of conventional measurement technique of the typedescribed in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of from 2 grams to 10grams” is inclusive of the endpoints, 2 grams and 10 grams, and all theintermediate values).

As used herein, approximating language may be applied to modify anyquantitative representation that may vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term or terms, such as “about” and “substantially,” maynot be limited to the precise value specified. The modifier “about”should also be considered as disclosing the range defined by theabsolute values of the two endpoints. For example, the expression “fromabout 2 to about 4″ also discloses the range from 2 to 4.”

It should be noted that many of the terms used herein are relativeterms. For example, the terms “upper” and “lower” are relative to eachother in location, i.e. an upper component is located at a higherelevation than a lower component in a given orientation, but these termscan change if the device is flipped. The terms “inlet” and “outlet” arerelative to a fluid flowing through them with respect to a givenstructure, e.g. a fluid flows through the inlet into the structure andflows through the outlet out of the structure. The terms “upstream” and“downstream” are relative to the direction in which a fluid flowsthrough various components, i.e. the flow fluids through an upstreamcomponent prior to flowing through the downstream component. It shouldbe noted that in a loop, a first component can be described as beingboth upstream of and downstream of a second component.

The terms “horizontal” and “vertical” are used to indicate directionrelative to an absolute reference, i.e. ground level. However, theseterms should not be construed to require structures to be absolutelyparallel or absolutely perpendicular to each other. For example, a firstvertical structure and a second vertical structure are not necessarilyparallel to each other. The terms “top” and “bottom” or “base” are usedto refer to surfaces where the top is always higher than the bottom/baserelative to an absolute reference, i.e. the surface of the earth. Theterms “upwards” and “downwards” are also relative to an absolutereference; an upwards flow is always against the gravity of the earth.

The present application refers to “the same order of magnitude.” Twonumbers are of the same order of magnitude if the quotient of the largernumber divided by the smaller number is a value less than 10.

Efficient separation technologies for multi-component liquid streamsthat eliminate any waste and reduce the required energy, and thereforepromote a sustainable environment, are needed. Large volume flow rateacoustophoretic phase separator technology using ultrasonic standingwaves provides the benefit of having no consumables, no generated waste,and a low cost of energy. The technology is efficient at removal ofparticles of greatly varying sizes, including separation of micron andsub-micron sized particles. Examples of acoustic filters/collectorsutilizing acoustophoresis can be found in commonly owned U.S. patentapplication Ser. Nos. 12/947,757; 13/085,299; 13/216,049; and13/216,035, the entire contents of each being hereby fully incorporatedby reference. Generally, an acoustophoretic system employs ultrasonicstanding waves to trap (i.e. hold stationary) secondary phase particles,gases, or liquids that are suspended in a host fluid stream. Thesecondary phase can be continuously separated out of the host fluid asthe mixture flows through the acoustophoretic system.

The platform technology described herein provides an innovative solutionthat includes a large volume flow rate acoustophoretic phase separatorbased on ultrasonic standing waves with the benefit of having noconsumables, no generated waste, and a low cost of energy.Acoustophoresis is a low-power, no-pressure-drop, no-clog, solid-stateapproach to particle removal from fluid dispersions: i.e., it is used toachieve separations that are more typically performed with porousfilters, but it has none of the disadvantages of filters. In particular,the present disclosure provides systems that operate at the macro-scalefor separations in flowing systems with high flow rates. The acousticresonator is designed to create a high intensity three dimensionalultrasonic standing wave that generates three dimensional pressuregradients and results in an acoustic radiation force that is larger thanthe combined effects of fluid drag and buoyancy or gravity, and istherefore able to trap (i.e., hold stationary) the suspended phase toallow more time for the acoustic wave to increase particleconcentration, agglomeration and/or coalescence. The present systemshave the ability to create ultrasonic standing wave fields that can trapparticles in flow fields with a linear velocity ranging from 0.1 mm/secto velocities exceeding 1 cm/s. This technology offers a green andsustainable alternative for separation of secondary phases with asignificant reduction in cost of energy. Excellent particle separationefficiencies have been demonstrated for particle sizes as small as onemicron.

This is an important distinction from previous approaches where particletrajectories were merely altered by the effect of the acoustic radiationforce. The scattering of the acoustic field off the particles results ina three dimensional acoustic radiation force, which acts as athree-dimensional trapping field. The acoustic radiation force isproportional to the particle volume (e.g. the cube of the radius) whenthe particle is small relative to the wavelength. It is proportional tofrequency and the acoustic contrast factor. It also scales with acousticenergy (e.g. the square of the acoustic pressure amplitude). Forharmonic excitation, the sinusoidal spatial variation of the force iswhat drives the particles to the stable positions within the standingwaves. When the acoustic radiation force exerted on the particles isstronger than the combined effect of fluid drag force andbuoyancy/gravitational force, the particle is trapped within theacoustic standing wave field. The action of the acoustic forces on thetrapped particles results in concentration, agglomeration and/orcoalescence of particles and droplets. Particles which are denser thanthe host fluid are separated through enhanced gravitational settling,and particles which are less dense than the host fluid are separatedthrough enhanced buoyancy.

Efficient and economic particle separation processes can be useful inmany areas of energy generation, e.g., producing water, hydro-fracking,and bio-fuels, e.g, harvesting and dewatering. Acoustophoretictechnology can be used to target accelerated capture of bacterial sporesin water, oil-recovery, and dewatering of bio-oil derived frommicro-algae. Current technology used in the oil recovery field does notperform well in recovery of small, i.e., less than 20 micron, oildroplets. However, the acoustophoretic systems described herein canenhance the capture and coalescence of small oil droplets, therebyshifting the particle size distribution resulting in an overallincreased oil capture. To be useful, it is generally necessary todemonstrate large flow rates at a level of 15-20 gallons per minute(GPM) per square foot (cross-sectional area). Another goal is theincreased capture of oil droplets with a diameter of less than 20microns. Much prior work on acoustophoretics only occurred at themicroscale, in MEMS applications in research settings. Industrialprocesses require high flow rates and continuous operation.

Acoustophoretic separation can also be used to aid such applications asadvanced bio-refining technology to convert low-cost readily availablenon-food biomass (e.g. municipal solid waste and sewage sludge) into awide array of chemicals and secondary alcohols that can then be furtherrefined into renewable gasoline, jet fuel, or diesel. A water treatmenttechnology is used to de-water the fermentation broth and isolatevaluable organic salts for further processing into fuels. The dewateringprocess is currently done through an expensive and inefficientultra-filtration method that suffers from frequent fouling of themembranes, a relatively low concentration factor, and a high capital andoperating expense. Acoustophoretic separation can filter out particleswith an incoming particle size distribution that spans more than threeorders of magnitude, namely from 600 microns to 0.3 microns, allowingimprovements in the concentration of the separated broth with a lowercapital and operational expense.

Acoustophoretic separation is also useful for the harvesting,oil-recovery, and dewatering of micro-algae for conversion into bio-oil.Current harvesting, oil recovery, and dewatering technologies formicro-algae suffer from high operational and capital expenses. Currentbest estimates put the price of a barrel of bio-oil derived frommicro-algae at a minimum of $200.00 per barrel. There is a need in theart of micro-algae biofuel for technologies that improve harvesting,oil-recovery, and dewatering steps of this process. Acoustophoreticseparation technology meets this need.

Some other applications are in the areas of wastewater treatment, greywater recycling, and water production. Other applications are in thearea of biopharmaceuticals, life sciences, and medical applications,such as the separation of lipids from red blood cells. This can be ofcritical importance during cardiopulmonary bypass surgery, whichinvolves suctioning shed mediastinal blood. Lipids are unintentionallyintroduced to the bloodstream when blood is re-transfused to the body.Lipid micro-emboli can travel to the brain and cause variousneuro-cognitive disorders. Therefore, there is a need to cleanse theblood. Existing methods are currently inefficient or harmful to redblood cells.

Particular embodiments focus on the capture and growth of sub 20 micronoil droplets. At least 80% of the volume of sub-20-micron droplets arecaptured and then grown to droplets that are bigger than 20 microns. Theprocess involves the trapping of the oil droplets in the acousticstanding wave, coalescence of many small trapped droplets, andeventually release of the larger droplets when the acoustic trappingforce becomes smaller than the buoyancy force.

Desirably, the ultrasonic transducers generate a three-dimensionalstanding wave in the fluid that exerts a lateral force on the suspendedparticles/secondary fluid to accompany the axial force so as to increasethe particle trapping capabilities of a acoustophoretic system. Typicalresults published in literature state that the lateral force is twoorders of magnitude smaller than the axial force. In contrast, thetechnology disclosed in this application provides for a lateral force tobe of the same order of magnitude as the axial force.

The present disclosure relates to the use of an acoustic standing wavegenerated by an ultrasonic transducer or transducers to separate oilfrom processed water on a macro scale. The oil may be partiallyemulsified with the water. The separation occurs by trapping the oilparticles at the pressure nodes and anti-pressure nodes in a standingwave. As the oil is trapped at these nodes, it agglomerates and, due tobuoyancy, will move to an area of trapped, concentrated oil. Thebuoyancy separation is accomplished through fluid dynamics with the mainfluid stream flowing in a downward direction and the trapped,agglomerated and coalesced oil particles floating upward, due tobuoyancy, into a trap.

The oil particles are separated from the fluid stream at theanti-pressure nodes of the acoustic standing wave due to the differencein their acoustic contrast factors from the fluid stream. The equationfor determining the acoustic contrast factor of an oil in a fluid isknown, and is related to the density of the fluid, the density of theoil in the fluid, the compressibility of the fluid, and thecompressibility of the oil in the fluid. Both oil and emulsified oiltypically have a negative contrast factor (φ).

In the present disclosure, a 3-D acoustic standing wave is generated bycausing the ultrasonic transducer to act in a “drumhead” fashion asopposed to a “piston” fashion. The “drumhead” operation of thepiezoelectric element in the ultrasonic transducer causes multiplestanding waves to be generated in a 3-D space. This is opposed to theaction of the piezoelectric crystal in the ultrasonic transducer actingin a “piston” fashion n where a single standing wave is produced.Through the use of a 3-D multi-standing wave, macro-scale trapping ofoil particles may be accomplished. This allows for high volumes ofprocessed water to be treated and the oil to be separated from thewater,

The piezoelectric crystal in the ultrasonic transducer may be directlyinterfaced with the fluid stream or may have a protective layer ormatching layer over the surface of the piezoelectric crystal that isinterfaced with the fluid stream, The protective layer may be a coating,such as a polyurethane or epoxy. The protective layer may also be platedonto the surface of the piezoelectric crystal that is interfaced withthe fluid stream. The plated layer may be added to the surface of thepiezoelectric crystal through either electrolytic or electrolessplating. The plating material may be nickel, chrome, copper, indium orcombination of layers of these materials. Also, the secondary materialor matching layer may be adhered to the surface of the piezoelectriccrystal such that the matching layer is now interfaced with the fluidstream. The matching layer may be a material such as a stainless steelthat is adhered to the piezoelectric crystal through the use of atwo-part epoxy system.

FIGS. 1-9 show various views of an acoustophoresis device of the presentdisclosure. Generally, the acoustophoresis device uses the ultrasonictransducer to separate suspended oil particles/droplets in a fluidstream into ordered, coalesced and agglomerated particles trapped in astanding wave of the acoustophoresis device. The flow of the fluidstream is from the upper end downward (i.e. with gravity). The fluidstream can enter the device through one of many inlets that surround acentral trapping device for the agglomerated and separated oil. Thefluid stream flows into the acoustophoresis separation device from apump through the inlet. The agglomerated and coalesced oil gainsbuoyancy and rises into the central oil trapping device. The device isshown here in an orientation where the flow direction is downwards,which is used for separating less-dense particles from the host fluid.However, the device may be essentially turned upside down to allowseparation of particles which are heavier than the host fluid. Insteadof a buoyant force in an upward direction, the weight of theagglomerated particles due to gravity pulls them downward.

The initial fluid stream is made up of a host fluid (e.g. water) and asuspended phase (e.g. oil droplets/articles). The fluid stream entersthe device 200 through one or more device inlets 206 into an annularplenum 220 at a first end 202 of the device. The first end 202 includesan outer sidewall 222 and an inner longitudinal sidewall 224. An endwall 212 is also visible, from which the longitudinal sidewall extends.The term “annular,” as used herein, only designates the area or volumebetween the outer sidewall and the inner longitudinal sidewall, andshould not be construed as requiring the first end of the device to havea circular cross-section. However, in contemplated embodiments the firstend of the device has a circular cross-section. The annular plenum hasan inner diameter 225 and an outer diameter 227. This constructionguides the fluid stream flow downwards in the direction of thecenterline, i.e. with little to no radial or circumferential motioncomponent. This helps to create laminar/plug flow later downstream. Onedevice inlet 206 is shown here, with three other inlets spaced about thefirst end being shown in dotted line. It is contemplated that any numberof inlets may be provided as desired. In particular embodiments, fourinlets are used. The inlets are radially oriented.

A contoured nozzle wall 230 reduces the outer diameter of the flow path,which generates higher velocities near the wall and reduces turbulence,producing near plug flow as the fluid velocity profile develops and thefluid passes through the connecting duct and into a flow/separationchamber. The contoured wall also adds a radial motion component to thesuspended particles, moving the particles closer to the centerline ofthe device and generating more collisions with rising, buoyantagglomerated particles. This radial motion will allow for optimumscrubbing of the particles from the fluid in the connecting duct priorto reaching the separation chamber. The term scrubbing is used todescribe the process of particle/droplet agglomeration, aggregation,clumping or coalescing, that occurs when a larger particle/droplettravels in a direction opposite to the fluid flow and collides withsmaller particles, in effect scrubbing the smaller particles out of thesuspension. The contoured nozzle wall directs the fluid in a manner thatgenerates large scale vortices at the entrance of the first deviceoutlet to also enhance particle collection. Generally, the flow area ofthe device is designed to be continually decreasing from the deviceinlets to the separation chamber to assure low turbulence and eddyformation for better particle separation, agglomeration, and collection.Put another way, the contoured wall 230 has a wide end 232 and a narrowend 234. The first end of the device/the wide end of the nozzle wall hasa first diameter 235, and the narrow end of the nozzle wall has a seconddiameter 237. The second diameter is less than the first diameter. Theconnecting duct 240 is downstream of the nozzle wall and connects to theinlet 256 of the flow chamber 250.

The flow/separation chamber 250 is downstream of the connecting duct 240and has an inlet 256 at a first end 252, and an outlet 258 at a secondend 254 opposite the first end. At least one ultrasonic transducer 270is present on a wall 260, and a reflector 272 is located on a wall 262opposite the transducer. Multiple transducers can be used, as desired.In use, standing waves are created between the transducer 270 andreflector 272. These standing waves can be used to agglomerateparticles, and this orientation is used to agglomerate particles thatare buoyant (e.g. oil). Fluid, containing residual particles, then exitsthrough the flow chamber outlet 258 and through a second device outlet210 located at a second end 204 of the device opposite the first end 202of the device. Also shown here is a transparent window 274 on a thirdwall 264 of the flow chamber. It is contemplated that in particularembodiments, the flow chamber has a rectangular cross-section. The flowchamber inlet and outlets have a circular cross-section for interfacingwith the other components of the device.

As the buoyant particles agglomerate, they eventually overcome thecombined effect of the fluid flow drag forces and acoustic radiationforce, and their buoyant force is sufficient to cause the buoyantparticles to rise upwards. In this regard, a first device outlet orcollection duct 208 is present at the first end of the device 202, andis surrounded by the longitudinal sidewall 224, or put another way isseparated from the device inlets 206 by the longitudinal sidewall 224,or put yet another way the first device outlet is a hole in the end wall212. The agglomerated buoyant particles exit the device through thefirst device outlet 208. The first device outlet and the second deviceoutlet are on opposite ends of the device.

It should be noted that the buoyant particles formed in the separationchamber 250 subsequently pass through the connecting duct 240. Thiscauses the incoming fluid stream flow from the device inlets 206 to flowover the rising agglomerated particles due to the inward radial motionimparted by the contoured wall 230. This allows the rising particles toalso trap smaller particles in the incoming flow, increasing scrubbingeffectiveness. The length of the connecting duct and the contourednozzle wall thus increase scrubbing effectiveness. Especially higheffectiveness is found for particles with a size of 0.1 microns to 10microns, where efficiency is very low for conventional methods. As notedhere, the distance from the device inlets 206 to the bottom of thelongitudinal sidewall 224 is marked as length (L). The first diameter ismarked as D1 (reference numeral 235). This length-to-diameter ratio here(i.e. L/D1) is less than 1.

The design here results in low flow turbulence at the flow chamberinlet, a scrubbing length before (i.e. upstream of) the flow chamber toenhance particle agglomeration and/or coalescence before acousticseparation, and the use of the collection vortices to aid particleremoval upstream of the flow chamber.

The ultrasonic transducer(s) are arranged to cover the entirecross-section of the fluid stream flowpath. In certain embodiments, theflow chamber has a square cross section of 6 inches×6 inches whichoperates at flow rates of up to 3 gallons per minute (GPM), or a linearvelocity of 8 mm/sec. The transducer can be a PZT-8 (Lead ZirconateTitanate) transducer with a 1-inch diameter and a nominal 2 MHzresonance frequency. Each transducer consumes about 28 W of power fordroplet trapping at a flow rate of 3 GPM. This translates in an energycost of 0.25 kW hr/m³. This is an indication of the very low cost ofenergy of this technology. Desirably, when multiple transducers arepresent, each transducer is powered and controlled by its own amplifier.This device shifts the particle size distribution in the host fluidthrough agglomeration of smaller oil droplets into larger oil droplets.

FIG. 10 is a cross-sectional diagram of a conventional ultrasonictransducer. This transducer has a wear plate/protective layer 50 at abottom end, epoxy layer 52, piezoelectric material 54 (made of, e.g.PZT), an epoxy layer 56, and a backing layer 58. The epoxy layer 56attaches backing layer 58 to the crystal 54. The entire assembly iscontained in a housing 60 which may be made out of, for example,aluminum. A connector 62 provides connection for wires to pass throughthe housing and connect to leads (not shown) which attach to thepiezoelectric material 54. Typically, backing layers are designed to adddamping and to create a broadband transducer with uniform displacementacross a wide range of frequency and are designed to suppress excitationat particular vibrational eigen-modes. Wear plates are usually designedas impedance transformers to better match the characteristic impedanceof the medium into which the transducer radiates, and face in thedirection in which the wave is generated. The piezoelectric material canbe, for example, a ceramic crystal.

FIG. 11 is a cross-sectional view of an ultrasonic transducer 81 of thepresent disclosure, which can be used with the acoustophoretic device ofFIGS. 1-9. Transducer 81 has an aluminum housing 82. A PZT crystal 86defines the bottom end of the transducer, and is exposed from theexterior of the housing. The crystal is supported on its perimeter bythe housing, with a small elastic layer, e.g. silicone or similarmaterial, located between the crystal and the housing.

Screws (not shown) attach an aluminum top plate 82 a of the housing tothe body 82 b of the housing via threads 88. The top plate includes aconnector 84 to pass power to the PZT crystal 86. The bottom and topsurfaces of the PZT crystal 86 each contain an electrode. A wrap-aroundelectrode tab 90 connects to the bottom electrode and is isolated fromthe top electrode. Electrical power is provided to the PZT crystal 86through the electrodes, with the wrap-around tab 90 being the groundconnection point. Note that the crystal 86 has no backing layer or epoxylayer as is present in FIG. 5. Put another way, there is an air gap 87in the transducer between aluminum top plate 82 a and the crystal 86. Aminimal backing may be provided in some embodiments.

The transducer design can affect performance of the system. A typicaltransducer is a layered structure with the ceramic crystal bonded to abacking layer and a wear plate. Because the transducer is loaded withthe high mechanical impedance presented by the standing wave, thetraditional design guidelines for wear plates, e.g., half wavelengththickness for standing wave applications or quarter wavelength thicknessfor radiation applications, and manufacturing methods may not beappropriate. Rather, in one embodiment of the present disclosure thetransducers, there is no wear plate or backing, allowing the crystal tovibrate in one of its eigenmodes with a high Q-factor. The vibratingceramic crystal/disk is directly exposed to the fluid flowing throughthe flow chamber.

Removing the backing (e.g. making the crystal air backed) also permitsthe ceramic crystal/piezoelectric material to vibrate higher order modesof vibration (e.g. higher order modal displacement) with little damping.In a transducer having a crystal with a backing, the crystal vibrateswith a more uniform displacement, like a piston. Removing the backingallows the crystal to vibrate in a non-uniform displacement mode. Thehigher order the mode shape of the crystal, the more nodal lines thecrystal has. The higher order modal displacement of the crystal createsmore trapping lines, although the correlation of trapping line to nodeis not necessarily one to one, and driving the crystal at a higherfrequency will not necessarily produce more trapping lines. In thepresent disclosure, the transducers are driven so that the piezoelectriccrystal vibrates in higher order modes of the general formula (m, n),where m and n are independently 1 or greater. In practice, thetransducers of the present disclosure will vibrate at higher orders than(1,2).

In some embodiments, the crystal may have a backing that minimallyaffects the Q-factor of the crystal (e.g. less than 5%). The backing maybe made of a substantially acoustically transparent material such asbalsa wood, foam, or cork which allows the crystal to vibrate in ahigher order mode shape and maintains a high Q-factor while stillproviding some mechanical support for the crystal. In anotherembodiment, the backing may be a lattice work that follows the nodes ofthe vibrating crystal in a particular higher order vibration mode,providing support at node locations while allowing the rest of thecrystal to vibrate freely. The goal of the lattice work or acousticallytransparent material is to provide support without lowering the Q-factorof the crystal or interfering with the excitation of a particular modeshape.

Placing the crystal in direct contact with the fluid also contributes tothe high Q-factor by avoiding the dampening and energy absorptioneffects of the epoxy layer and the wear plate. Other embodiments mayhave wear plates or a wear surface/protective layer to prevent the PZT,which contains lead, contacting the host fluid. This may be desirablein, for example, biological applications such as separating blood. Suchapplications might use a wear layer such as chrome, electrolytic nickel,or electroless nickel. Chemical vapor deposition could also be used toapply a layer of poly(p-xylxyene) (e.g. Parylene) or other polymer.Organic and biocompatible coatings such as silicone or polyurethane arealso contemplated for use as a wear surface.

FIG. 12 illustrates two different ultrasonic transducers that can beused in the devices of the present disclosure. The transducer on theright shows a circular-shaped PZT-8 crystal 110 that is 1 inch indiameter. The transducer on the right shows a rectangular-shapedcrystal, which here is a square 1 inch by 1 inch crystal. The effect oftransducer shape on oil separation efficiency was investigated, andTable 1 shows the results.

TABLE 1 Results of Investigation of Round and Square Transducer ShapeTotal Power Transducer Input Flowrate Duration Capture Efficiency Shape(Watts) (ml/min) (min) (%) Round 20 500 45 59% Square 20 500 30 91%

The results indicate that the square transducer 112 provides better oilseparation efficiencies than the round transducer 110, explained by thefact that the square transducer 112 provides better coverage of the flowchannel with acoustic trapping forces, and that the round transduceronly provides strong trapping forces along the centerline of thestanding wave.

The size, shape, and thickness of the transducer determine thetransducer displacement at different frequencies of excitation, which inturn affects oil separation efficiency. Typically, the transducer isoperated at frequencies near the thickness resonance frequency (halfwavelength). Gradients in transducer displacement typically result inmore places for oil to be trapped. Higher order modal displacementsgenerate three-dimensional acoustic standing waves with strong gradientsin the acoustic field in all directions, thereby creating equally strongacoustic radiation forces in all directions, leading to multipletrapping lines, where the number of trapping lines correlate with theparticular mode shape of the transducer.

FIG. 13 shows the measured electrical impedance amplitude of thetransducer as a function of frequency in the vicinity of the 2.2 MHztransducer resonance. The minima in the transducer electrical impedancecorrespond to acoustic resonances of the water column and representpotential frequencies for operation. Numerical modeling has indicatedthat the transducer displacement profile varies significantly at theseacoustic resonance frequencies, and thereby directly affects theacoustic standing wave and resulting trapping force. Since thetransducer operates near its thickness resonance, the displacements ofthe electrode surfaces are essentially out of phase. The typicaldisplacement of the transducer electrodes is not uniform and variesdepending on frequency of excitation. As an example, at one frequency ofexcitation with a single line of trapped oil droplets, the displacementhas a single maximum in the middle of the electrode and minima near thetransducer edges. At another excitation frequency, the transducerprofile has multiple maxima leading to multiple trapped lines of oildroplets. Higher order transducer displacement patterns result in highertrapping forces and multiple stable trapping lines for the captured oildroplets.

To investigate the effect of the transducer displacement profile onacoustic trapping force and oil separation efficiencies, an experimentwas repeated ten times, with all conditions identical except for theexcitation frequency. Ten consecutive acoustic resonance frequencies,indicated by circled numbers 1-9 and letter A on FIG. 13, were used asexcitation frequencies. The conditions were an experiment duration of 30min, a 1000 ppm oil concentration, a flow rate of 500 ml/min, and anapplied power of 20 W.

As the emulsion passed by the transducer, the trapping lines of oildroplets were observed and characterized. The characterization involvedthe observation and pattern of the number of trapping lines across thefluid channel, as shown in FIG. 14, for seven of the ten resonancefrequencies identified in FIG. 13.

The effect of excitation frequency clearly determines the number oftrapping lines, which vary from a single trapping line at the excitationfrequency of acoustic resonance 5 and 9, to nine trapping lines foracoustic resonance frequency 4. At other excitation frequencies four orfive nodal trapping lines are observed. Different displacement profilesof the transducer can produce different (more) trapping lines of thestanding waves, with more gradients in displacement profile generallycreating higher trapping forces and more trapping lines.

Table 2 summarizes the findings from an oil trapping experiment using asystem similar to FIGS. 1-9. An important conclusion is that the oilseparation efficiency of the acoustic separator is directly related tothe mode shape of the transducer. Higher order displacement profilesgenerate larger acoustic trapping forces and more trapping linesresulting in better efficiencies. A second conclusion, useful forscaling studies, is that the tests indicate that capturing 5 micron oildroplets at 500 ml/min requires 10 Watts of power per square-inch oftransducer area per 1″ of acoustic beam span. The main dissipation isthat of thermo-viscous absorption in the bulk volume of the acousticstanding wave. The cost of energy associated with this flow rate is0.667 kWh per cubic meter.

TABLE 2 Trapping Pattern Capture Efficiency Study Resonance Total Power# of Capture Peak Input Trapping Flowrate Duration Efficiency Location(Watts) Lines (ml/min) (min) (%) 4 20 9 500 30 91% 8 20 5 500 30 58% A20 4 500 30 58% 9 20 2 500 30 37%

In larger systems, different transducer arrangements are feasible. FIG.15 shows a transducer array 120 including three square 1″×1″ crystals120 a, 120 b, 120 c. Two squares are parallel to each other, and thethird square is offset to form a triangular pattern and get 100%acoustic coverage. FIG. 16 shows a transducer array 122 including tworectangular 1″×2.5″ crystals 122 a, 122 b arranged with their long axesparallel to each other. Power dissipation per transducer was 10 W per1″×1″ transducer cross-sectional area and per inch of acoustic standingwave span in order to get sufficient acoustic trapping forces. For a 4″span of an intermediate scale system, each 1″×1″ square transducerconsumes 40 W. The larger 1″×2.5″ rectangular transducer uses 100 W inan intermediate scale system. The array of three 1″×1″ squaretransducers would consume a total of 120 W and the array of two 1″×2.5″transducers would consume about 200 W. Arrays of closely spacedtransducers represent alternate potential embodiments of the technology.Transducer size, shape, number, and location can be varied as desired togenerate desired three-dimensional acoustic standing waves.

FIG. 17 is a computer model of an acoustophoretic separator 92 simulatedto produce FIGS. 18-29. The piezo ceramic crystal 94 is in directcontact with the fluid in the water channel 96. A layer of silicon 98 isbetween the crystal 94 and the aluminum top plate 100. A reflector 102reflects the waves to create standing waves. The reflector is made of ahigh acoustic impedance material such as steel or tungsten, providinggood reflection. For reference, the Y-axis 104 will be referred to asthe axial direction. The X-axis 106 will be referred to as the radial orlateral direction. The acoustic pressure and velocity models werecalculated in COMSOL including piezo-electric models of the PZTtransducer, linear elastic models of the surrounding structure (e.g.reflector plate and walls), and a linear acoustic model of the waves inthe water column. The acoustic pressure and velocity was exported asdata to MATLAB. The radiation force acting on a suspended particle wascalculated in MATLAB using Gor'kov's formulation. The particle and fluidmaterial properties, such as density, speed of sound, and particle size,are entered into the program, and used to determine the monopole anddipole scattering contributions. The acoustic radiation force isdetermined by performing a gradient operation on the field potential U,which is a function of the volume of the particle and the time averagedpotential and kinetic energy of the acoustic field.

FIGS. 18-21 show simulations of the difference in trapping pressuregradients between a single acoustic wave and a multimode acoustic wave.FIG. 18 shows the axial force associated with a single standing acousticwave. FIG. 19 shows the lateral force due to a single standing acousticwave. FIG. 20 and FIG. 21 show the axial force and lateral force,respectively, in a multi-mode (higher order vibration modes havingmultiple nodes) piezoelectric crystal excitation where multiple standingwaves are formed. The electrical input is the same as the single mode ofFIG. 18 and FIG. 19, but the trapping force (lateral force) is 70 timesgreater (note the scale to the right in FIG. 19 compared to FIG. 21).The figures were generated by a computer modeling simulation of a 1 MHzpiezo-electric transducer driven by 10 V AC potted in an aluminum topplate in an open water channel terminated by a steel reflector (see FIG.17). The field in FIG. 18 and FIG. 19 is 960 kHz with a peak pressure of400 kPa. The field in FIG. 20 and FIG. 21 is 961 kHz with a peakpressure of 1400 kPa. In addition to higher forces, the 961 kHz fieldhas more gradients and focal spots.

FIG. 22 shows a three dimensional computer generated model of a modeshape calculation showing the out-of-plane displacement for a circularcrystal driven at a frequency of 1 MHz.

FIGS. 23-29 are based on the model of FIG. 17 with a PZT-8piezo-electric transducer operating at 2 MHz. The transducer is 1″ wideand 0.04″ thick, potted in an aluminum top plate (0.125″ thick) in a4″×2″ water channel terminated by a steel reflector plate (0.180″thick). The acoustic beam spans a distance of 2″. The depth dimension,which is 1″, is not included in the 2D model. The transducer is drivenat 15V and a frequency sweep calculation is done to identify the variousacoustic resonances. The results of the three consecutive acousticresonance frequencies, i.e., 1.9964 MHz (FIGS. 23-25), 2.0106 MHz (FIG.26 and FIG. 27), and 2.025 MHz (FIG. 28 and FIG. 29), are shown. Theacoustic radiation force is calculated for an oil droplet with a radiusof 5 micron, a density of 880 kg/m³, and speed of sound of 1700 m/sec.Water is the main fluid with a density of 1000 kg/m³, speed of sound of1500 m/sec, and dynamic viscosity of 0.001 kg/msec.

FIG. 23 shows the lateral (horizontal) acoustic radiation force. FIG. 24shows the axial (vertical) component for a resonance frequency of 1.9964MHz. FIG. 25 shows the acoustic pressure amplitude. FIG. 23 and FIG. 24show that the relative magnitude of the lateral and axial component ofthe radiation force are very similar, about 1.2e-10 N, indicating thatit is possible to create large trapping forces, where the lateral forcecomponent is of similar magnitude or higher than the axial component.This is a new result and contradicts typical results mentioned in theliterature.

A second result is that the acoustic trapping force magnitude exceedsthat of the fluid drag force, for typical flow velocities on the orderof mm/s, and it is therefore possible to use this acoustic field to trapthe oil droplet. Of course, trapping at higher flow velocities can beobtained by increasing the applied power to the transducer. That is, theacoustic pressure is proportional to the driving voltage of thetransducer. The electrical power is proportional to the square of thevoltage.

A third result is that at the frequency shown, high trapping forcesassociated with this particular trapping mode extend across the entireflow channel, thereby enabling capture of oil droplets across the entirechannel width. Finally, a comparison of the minima of the acoustictrapping force field, i.e., the locations of the trapped particles, withthe observed trapping locations of droplets in the standing wave showsgood agreement, indicating that COMSOL modeling is indeed an accuratetool for the prediction of the acoustic trapping of particles. This willbe shown in more detail below.

FIG. 26 shows the lateral force component at a resonance frequency of2.0106 MHz, and FIG. 27 shows the axial acoustic radiation forcecomponent at a resonance frequency of 2.0106 MHz. FIG. 26 and FIG. 27exhibit higher peak trapping forces than FIG. 23 and FIG. 24. Thelateral acoustic radiation forces exceed the axial radiation force.However, the higher trapping forces are located in the upper part of theflow channel, and do not span the entire depth of the flow channel. Itwould therefore represent a mode that is effective at trapping particlesin the upper portion of the channel, but not necessarily across theentire channel. Again, a comparison with measured trapping patternsindicates the existence of such modes and trapping patterns.

FIG. 28 shows the lateral force component at a resonance frequency of2.025 MHz, and FIG. 29 shows the axial acoustic radiation forcecomponent at a resonance frequency of 2.025 MHz. The acoustic fieldchanges drastically at each acoustic resonance frequency, and thereforecareful tuning of the system is critical. At a minimum, 2D models arenecessary for accurate prediction of the acoustic trapping forces.

2D axisymmetric models were developed to calculate the trapping forcesfor circular transducers. The models were used to predict acoustictrapping forces on particles, which can then be used to predict particletrajectories in combination with the action of fluid drag and buoyancyforces. The models clearly show that it is possible to generate lateralacoustic trapping forces necessary to trap particles and overcome theeffects of buoyancy and fluid drag. The models also show that circulartransducers do not provide for large trapping forces across the entirevolume of the standing wave created by the transducer, indicating thatcircular transducers only yield high trapping forces near the center ofthe ultrasonic standing wave generated by the transducer, but providemuch smaller trapping forces toward the edges of the standing wave. Thisfurther indicates that the circular transducer only provides limitedtrapping for a small section of the fluid flow that would flow acrossthe standing wave of the circular transducer, and no trapping near theedges of the standing wave.

FIG. 30 is a picture showing the separation attained by an apparatus ofFIGS. 1-9 after 30 minutes of operation. This picture is taken in acolumn attached to the first device outlet. An air layer is present atthe top, followed by an oil layer and a water column. The oil is clearlyseparated from the water column.

The acoustophoretic devices of the present disclosure create a threedimensional pressure field which includes standing waves perpendicularto the fluid flow. The pressure gradients are large enough to generateacoustophoretic forces orthogonal to the standing wave direction (i.e.,the acoustophoretic forces are parallel to the fluid flow direction)which are of the same order of magnitude as the acoustophoretic forcesin the wave direction. This permits better particle trapping andcollection in the flow chamber and along well-defined trapping lines, asopposed to merely trapping particles in collection planes as inconventional devices. The particles have significant time to move tonodes or anti-nodes of the standing waves, generating regions where theparticles can concentrate, agglomerate, and/or coalesce.

In some embodiments, the fluid flow has a Reynolds number of up to 500,i.e. laminar flow is occurring. For practical application in industry,the Reynolds number is usually from 10 to 500 for the flow through thesystem. The particle movement relative to the fluid motion generates aReynolds number much less than 1.0. The Reynolds number represents theratio of inertial flow effects to viscous effects in a given flow field.For Reynolds numbers below 1.0, viscous forces are dominant in the flowfield. This results in significant damping where shear forces arepredominant throughout the flow. This flow where viscous forces aredominant is called Stokes flow. The flow of molasses is an example.

Wall contouring and streamlining have very little importance to the flowof very viscous fluids or the flow in very tiny passages, like MEMSdevices. The flow of the particles relative to the fluid in MEMS deviceswill be Stokes flow because both the particle diameters and the relativevelocities between the particles and fluid are very small. On the otherhand, the Reynolds number for the flow through the present system willbe much greater than 1.0 because the fluid velocity and inlet diameterare much larger. For Reynolds numbers much greater than 1.0, viscousforces are dominant only where the flow is in contact with the surface.This viscous region near the surface is called a boundary layer and wasfirst recognized by Ludwig Prandtl (Reference 2). In duct flow, the flowwill be laminar if the Reynolds number is significantly above 1.0 andbelow 2300 for fully developed flow in the duct. The flow velocitystarts off uniform. As the flow moves down the duct, the effect of wallviscous forces will diffuse inward towards the centerline to generate aparabolic velocity profile. This parabolic profile will have a peakvalue that is twice the average velocity. The length required for theparabolic profile to develop is a function of the Reynolds number. For aReynolds number of 20, the development length will be 1.2 ductdiameters. Thus, fully developed flow happens very quickly. This peakvelocity in the center can be detrimental to acoustic particleseparation. Also, turbulence can occur and so flow surface contouring isvery important in controlling the flow. Thus, the shape of the contourednozzle wall will have a large effect on the final velocity profile. Thearea convergence increases the flow average velocity, but it is the wallcontour that determines the velocity profile. The nozzle wall contourwill be a flow streamline, and is designed with a small radius ofcurvature.

The transducer(s) is/are used to create a pressure field that generatesforces of the same order of magnitude both orthogonal to the standingwave direction and in the standing wave direction. When the forces areroughly the same order of magnitude, particles of size 0.1 microns to300 microns will be moved more effectively towards regions ofagglomeration (“trapping lines”). Because of the equally large gradientsin the orthogonal acoustophoretic force component, there are “hot spots”or particle collection regions that are not located in the regularlocations in the standing wave direction between the transducer and thereflector. Such hot spots are located in the maxima or minima ofacoustic radiation potential. Such hot spots represent particlecollection locations which allow for better wave transmission betweenthe transducer and the reflector during collection and strongerinter-particle forces, leading to faster and better particleagglomeration.

In biological applications, many parts, e.g. the tubing leading to andfrom the device, may all be disposable, with only the transducer andreflector to be cleaned for reuse. Avoiding centrifuges and filtersallows better separation of cells without lowering the viability of thecells. The form factor of the acoustophoretic device is also smallerthan a filtering system, allowing cell separation to be miniaturized.The transducers may also be driven to create rapid pressure changes toprevent or clear blockages due to agglomeration of cells. The frequencyof the transducers may also be varied to obtain optimal effectivenessfor a given power.

The present disclosure has been described with reference to exemplaryembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the present disclosure be construed asincluding all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. An acoustophoresis device, comprising: one or more device inlets at afirst end of the device, the first end having a first diameter forreceiving fluid flow; a contoured wall downstream of the inlet thatnarrows the fluid flow to a second diameter of a connecting duct; a flowchamber downstream of the connecting duct, the flow chamber having: aninlet at a first end for receiving the fluid flow, an outlet at a secondend opposite the first end, at least one ultrasonic transducer locatedon a wall of the flow chamber, the ultrasonic transducer including apiezoelectric material driven by a voltage signal to create amulti-dimensional standing wave in the flow chamber, and a reflectorlocated on a wall on the opposite side of the flow chamber from the atleast one ultrasonic transducer; a first device outlet located at thefirst end of the device and separated from the device inlet by alongitudinal sidewall; and a second device outlet located at a secondend of the device downstream of the flow chamber outlet.
 2. The deviceof claim 1, wherein the device includes a plurality of device inletsspaced about the first end of the device, and the longitudinal sidewallis spaced apart from the contoured wall.
 3. The device of claim 1,wherein the piezoelectric material of the at least one ultrasonictransducer has a rectangular shape.
 4. The device of claim 1, whereinthe reflector has a non-planar surface.
 5. The device of claim 1,wherein the first end of the device has a circular cross-section and theflow chamber has a rectangular cross-section.
 6. The device of claim 1,wherein the multi-dimensional standing wave results in an acousticradiation force having an axial force component and a lateral forcecomponent that are of the same order of magnitude.
 7. The device ofclaim 1, wherein the transducer comprises: a housing having a top end, abottom end, and an interior volume; and a crystal at the bottom end ofthe housing having an exposed exterior surface and an interior surface,the crystal being able to vibrate when driven by a voltage signal. 8.The device of claim 1, wherein no backing layer is present within thehousing of the transducer, and an air gap is present in the interiorvolume between the crystal and a top plate at the top end of thehousing.
 9. The device of claim 1, wherein the transducer furthercomprises a backing layer contacting the interior surface of thecrystal, the backing layer being made of a substantially acousticallytransparent material.
 10. The device of claim 9, wherein thesubstantially acoustically transparent material is balsa wood, cork, andfoam.
 11. The device of claim 9, wherein the substantially acousticallytransparent material has a thickness of up to 1 inch.
 12. The device ofclaim 1, wherein the flow chamber further comprises a transparent windowfor viewing the interior of the flow chamber.
 13. The device of claim 1,wherein the device has a length L from the at least one device inlet toa bottom of the longitudinal sidewall, and a ratio of the length L tothe first diameter is less than
 1. 14. The device of claim 1, whereinthe flow chamber has a plurality of the ultrasonic transducers locatedon the wall of the flow chamber.